Hi Tb

. Need some help and advice regarding running and extension cab off of a Combo amp. (I apologize in advance for my terrible electronics knowledge)
The Amp i currently use is a Hartke 210C (Not the 5210 with the LH Series head built into it). The guitarist in my band has gone and got himself a Marshall cab and head and our gigs are starting to get bigger so i need to size up too.
The head has one output and underneath it says a minimum of 4 ohm. So im presuming that if i was to buy an 8ohm Hydrive 410 to use as an extension, the head would work in the combo driving the cabinet like a standard head and cab. Or if the combo speaker set was 8ohm, i could buy a splitter, wire them in parallel therefore the output is 4ohms and i have a 6x10 rig? Or is this incorrect?